In a hydrofoil seacraft, the hull of the craft is lifted out of the water by means of foils which are carried on struts and usually pass through the water beneath the surface thereof. In passing through the water, and assuming that sufficient speed is attained, the foils create enough lift to raise the hull above the surface and hence eliminate the normal resistance encountered by a ship hull in passing through the water.
In the usual case, there are forward and aft foils both provided with control flaps similar to those used on aircraft, although in some cases the entire foils may be rotatable and used as control surfaces. The other essential control element is the rudder which pierces or is submerged beneath the surface of the water and may be either forward or aft of the craft depending upon its design. In most hydrofoils, the flaps or control surfaces are used primarily to cause the craft to ascend or descend and to control the craft about its pitch and roll axes. However, they can also be used in combination with the rudder to bank the ship about its roll axis during a turn. The flaps are also used to stabilize the craft during movement on water, so that pitching or rolling motions can be minimized by proper counterbalancing movement of the flaps. A control system of this type is disclosed and claimed in a copending application of D. R. Stark et al, Ser. No. 302,599, filed Oct. 31, 1972, which matured into U.S. Pat. No. 3,886,884 on June 3, 1975 and is assigned to the Assignee of the present invention. The invention described herein is particularly useful in a system of this type although its usefulness is not necessarily restricted to this particular system.
In any hydrofoil control system, safety is a paramount consideration. In the type of control mentioned above, the roll control system comprises sensing means for sensing motion of the craft about the roll axis and means responsive to the signals generated by this sensing means to effect the desired movement of the control surfaces to counteract the rolling motion and stabilize the craft about the roll axis. In case of a failure or malfunction in this roll control system, an unsafe condition can develop in which the motion of the craft becomes unstable, and the craft can exhibit divergent motions such that it may impact the water with angular rates of motion and attitudes that can endanger personnel on board the craft and cause possible damage to the ship. It would be desirable, therefore, to detect the occurrence of such a failure promptly and to cause the ship to land, that is, to descend from the foil-borne mode to the hull-borne mode of operation, before these potentially dangerous conditions can result.